1. Field of the Invention
The present invention relates to surface-mount electronic components, and more particularly relates to a surface-mount electronic component having a terminal electrode film formed by a film-forming processing such as plating.
2. Description of the Related Art
FIG. 11 shows one example of a conventional surface-mount electronic component. A surface-mount electronic component 60 includes a rectangular main unit 11 having three terminal electrode films 12 to 14 provided on the surface thereof by electroless plating or electroplating. The terminal electrode films 12 and 14 are provided on the corresponding ends of the main unit 11 and function as an input terminal electrode film 12 is located between the terminal electrode films 12 and 14 and functions as a ground terminal.
The main unit 11 is formed by adhering a piezoelectric substrate 15, an overlaid ceramic cover member 16, and an underlaid ceramic cover member 17. The major surfaces of the piezoelectric substrate 15 facing each other have corresponding vibrating electrodes 21 and 22 provided thereon, as shown in FIG. 12. These vibrating electrodes 21 and 22 function as internal electrodes and constitute, along with the piezoelectric substrate 15, a piezoelectric resonator 230. As shown in FIG. 11, a lead-in terminal 21a of the vibrating electrode 21 extends to the right end portion of the piezoelectric substrate 15 and is exposed at the surface of the main unit 11, including a right end surface 11a. A lead-in terminal 22a of the vibrating electrode 22 extends to the left end portion of the piezoelectric substrate 15 and is exposed at the surface of the main unit 11, including a left end surface 11b. 
The terminal electrode film 12 is electrically connected to the lead-in terminal 21a, and the terminal electrode film 14 is electrically connected to the lead-in terminal 22a. 
Thus, the surface-mount electric component 60 is constructed as a three-terminal electronic component (an oscillator) having an equivalent circuit as shown in FIG. 13. That is, the piezoelectric resonator 230 is connected between the terminal electrode film 12 (the input terminal) and the terminal electrode film 14 (the output terminal) capacitor C1 is located between the terminal electrode films 12 and 13 (the ground terminal), and is coupled across the terminal electrode films 12 and 13. A capacitor C2 is located between the terminal electrode films 13 and 14, and is coupled across the terminal electrode films 13 and 14.
In the conventional surface-mount electronic component 60, as shown in FIGS. 11 and 12, the lead-in terminals 21a and 22a are exposed at the end surfaces 11a and 11b of the main unit 11, respectively. Accordingly, when the terminal electrodes 12 to 14 are formed by electroless plating or electroplating, a plated film 26 is formed on the surface of each of the exposed lead-in terminals 21a and 22a. 
However, since each of the lead-in terminals 21a and 22a is thin, the area for adhesion between the plated film 26 and the lead-in terminals 21a or 22a is very small. Therefore, the adhesion therebetween is weak, which causes the plated film 26 to be easily separated from the lead-in terminal 21a or 22a. For example, when some force or impact is applied to the plated film 26 in the course of forming the terminal electrodes 12 to 14, or some tension is caused when molten solder is applied to the plated film 26 in the course of mounting of the printed substrate, the plated film 26 can easily separate from the plated film 26. At this time, when the separated plated film 26 comes into contact with, for example, the terminal electrode film 12 or 13, as shown in FIG. 11, short-circuiting occurs between the terminal electrode films 12 and 13.
In order to overcome the problems described above, preferred embodiments of the present invention provide a highly reliable surface-mount electronic component which is constructed so as to prevent short-circuiting between terminal electrode films using films formed by various film-forming processes such as plating.
According to one preferred embodiment of the present invention, a surface-mount electronic component includes a terminal electrode film that is formed by film-forming processing on the surface of a main unit of the surface-mount electronic component, and a lead-in terminal conducted from an internal electrode disposed in the surface-mount electronic component and extending up to the surface of the main unit for establishing electrical connection between the internal electrode and the terminal electrode film. In the surface-mount electronic component, the lead-in terminal of the internal electrode is conducted up to at least one of the surfaces of the main unit, except a surface-mount surface of the main unit and the surface opposite to the surface-mount surface, and an exposed portion of the lead-in terminal is coated by at least one of the terminal electrode film and a protective film. In this context, the meaning of xe2x80x9ccoatedxe2x80x9d includes a case in which only a portion of an exposed part is coated.
A portion of the lead-in terminal exposed at the surface of the main unit is coated by the terminal electrode film and the protective film. When the terminal electrode film is formed by plating, the coating of the terminal electrode film or the protective film does not allow the plating to form on the exposed part of the lead-in terminal. Accordingly, easily separable and useless plated film is not formed.
In the surface-mount electronic component, alternatively, the surface-mount electronic component has at least two terminal electrode films, and the length of the exposed portion of each of the at least two terminal electrode films which is coated by none of the at least two terminal electrode films and the protective film, is shorter than the distance between the at least two terminal electrode films.
With the above-described setting, the length of the useless plated film, which is formed on each of the exposed parts of the lead-in terminals, and which is coated by none of the terminal electrode films and the protective film, is shorter than the distance between two adjacent terminal electrode films. Accordingly, even though the useless plated film is separated from the exposed part of the lead-in terminal, this separated plated film cannot cause short-circuiting to occur between the two adjacent terminal electrode films.
Other features, characteristics, elements and advantages of preferred embodiments of the present invention will be apparent from the detailed description of preferred embodiments of the present invention with reference to the attached drawings.